The term “refractory products” includes fused cast products and sintered products.
In contrast to sintered products, fused cast products usually include a very substantial intergranular vitreous phase that fills an array of crystallized grains. The problems encountered by sintered products and by fused cast products in their respective applications, and the technical solutions adopted to overcome them are thus generally different. Further, because of the major differences between their production methods, a composition developed to produce a fused cast product cannot, a priori, be used as is to produce a sintered product, and vice versa.
Sintered products are obtained by mixing appropriate starting materials then shaping that green mixture and firing the resulting green body at a temperature and for a time sufficient to sinter that green body.
Depending on their chemical composition, sintered products have a variety of properties and are thus intended for a wide variety of industries.
One highly specific application is the use of sintered products, conventionally in the form of beads, as a milling medium, in particular to finely mill mineral, inorganic, or organic substances. In that application, the beads are dispersed in an aqueous medium or a solvent at a temperature that may exceed 80° C., and then they are subjected to friction by contact with the substance to be milled, by mutual contact, and by contact with the elements of the mill. Hence, the service life of the beads is directly dependent on their wear resistance in that aqueous medium or solvent.
US 2009/0036291 (or WO 2009/018024) discloses sintered beads based on zirconia and alumina that include approximately 9% to 12% of cerium oxide, CeO2, and in the range 0.01% to 0.8% of calcium oxide, CaO. It compares them with CeO2-TZP beads (“CeO2-stabilized tetragonal zirconia polycrystals”), but considers, on the basis of historical data, that there is no need to compare them with Y-TZP beads, known for their poor performance in aqueous media.
Modifying the composition of a sintered bead, especially by incorporating a new oxide, even in a small quantity, may have very substantial consequences on its properties. It is also known that the quantity of the stabilized phase of a zirconia may be modified by heat treating or mechanically treating that zirconia.
In order to increase the yields of milling operations, the milling particles have to be ever more wear resistant, while having high resistance to degradation in a hot liquid medium, in particular when they are in contact with water at more than 80° C.; below, such conditions are termed “hydrothermal conditions”.
Further, there is a constant drive towards increasing milling yield, i.e. obtaining a maximum quantity of milled material for a given cost.
One aim of the invention is to satisfy those needs, at least in part.